1. Field of the Invention
This invention relates to windings for switched reluctance machines. The invention is particularly applicable to windings for single-phase machines.
2. Description of Related Art
Switched reluctance machines, in which the position of the rotor is used to switch the excitation of the windings, are now well-established for many applications. The characteristics of such switched reluctance machines are described in, for example, xe2x80x9cThe characteristics, design and application of switched reluctance motors and drivesxe2x80x9d by Stephenson and Blake, PCIM ""93, Nxc3xcrnberg, Jun. 21-24, 1993, incorporated herein by reference. The present invention is generally applicable to switched reluctance machines operating as motors or generators.
FIG. 1 shows the principal components of a typical switched reluctance drive system. The input DC power supply 11 can be either a battery or rectified and filtered AC mains. The DC voltage provided by the power supply 11, which can be of constant or varying magnitude depending on the configuration chosen, is switched across the phase windings 16 of the motor 12 by a power converter 13 under the control of the electronic control unit 14. The switching must be correctly synchronized to the angle of rotation of the rotor for proper operation of the drive. A rotor position detector 15 is typically employed to supply signals corresponding to the angular position of the rotor. The output of the rotor position detector 15 may also be used to generate a speed feedback signal.
The rotor position detector 15 may take many forms, for example it may take the form of hardware, as shown schematically in FIG. 1, or of a software algorithm which calculates the position from other monitored parameters of the drive system, as described in EP-A-0573198 (Ray), incorporated herein by reference.
Some form of current feedback signal 18, using either an isolated or a non-isolated current transducer 19, is generally provided to the controller from either the phase winding(s) or the DC link.
FIG. 2 shows a cross-section of a prior art, 3-phase, switched reluctance machine in which the stator has six poles and the rotor has four poles. Each stator pole 21 carries a coil 23. Each of the coil sides extends along the space between adjacent stator poles. The coil extends around the stator pole at each end in the form of a semi-circular end portion which lies beyond the axial extent of the stator pole. The coils on diametrically opposite poles are connected in series or parallel to form one of phase windings 16. The stator poles in FIG. 2 are labeled to illustrate the three phases A, B, and C. The rotor poles 24 protrude from a rotor body 26. The rotor is mounted on a shaft 28 to rotate co-axially in the bore defined by the faces of the stator poles. Typically, both stator and rotor are formed by stacking laminations of electrical sheet steel and securing them together in known ways.
FIG. 3 generally shows typical switching circuitry in the power converter 13 that controls the energization of the phase winding 16. When switches 31 and 32 are closed, the phase winding is coupled to the source of DC power and is energized. Many other configurations of switching circuitry are known in the art: some of these are discussed in the Stephenson and Blake paper cited above.
The conventional method of manufacture of such motors is to produce laminations from electrical sheet steel and then to consolidate the required number of these laminations by one of a number of known methods. This produces a rotor stack, which is then mounted on a shaft, and a stator stack which can receive the coils.
Typically one coil is arranged around each pole, as shown in FIG. 2, though methods of using fewer coils are known. For example, U.S. Pat. No. 5,654,601 (Fulton), incorporated herein by reference, discloses a method of using coils only on alternate poles, and U.S. Pat. No. 5,043,618 (Stephenson), incorporated herein by reference, discloses a method of grouping two or more poles of like polarity under one coil. Having connected the coils to form the correct number of phases and flux patterns, the winding may be impregnated with a varnish to improve its reliability and thermal performance. End frames or a housing may then be added to produce a stand-alone assembly which can be tested for performance compliance, or assembled into a larger component.
FIGS. 4(a) and (b) show an end view and an axial cross section, respectively, of a stator 40 and rotor 42 having laminations suitable for use in accordance with U.S. Pat. No. 5,043,618. FIGS. 5(a) and (b) show two coils 51, 52 inserted in the stator 40 of FIG. 4(a) according to the prior art. It will be noted that each coil 51, 52 embraces two stator poles and that their coil sides 53, 54 (i.e. that portion of the coil lying between the axial extremes of the stator) both share a slot with a coil side of the other coil. Further, it will be seen that the two end windings 55, 56 of each coil (i.e. that portion of the coil lying outside the stator) lie on the same side of the stator axis. It will be appreciated that the end windings are formed in this way in order to allow the rotor to be assembled into the completed stator assembly. To produce this shape, the coil can be wound in a flat plane, inserted into the stator and pressed into shape. Alternatively, it must be wound in situ, with each turn being formed to the final shape as the winding proceeds.
In an attempt to reduce the cost of drive systems, it is becoming increasingly common to integrate the power switches and electronic control unit into the housing of the motor, thus reducing the number of separate assemblies required. This entails connecting the winding(s) of the machine to the power switches after the electronic components have been assembled to the housing of the machine. Only then can the complete drive system be tested. It would be more convenient to assemble the windings to the electronic sub-assemblies and test that complete unit before assembling the winding into the stator.
It is well-known that some forms of synchronous and induction motors, especially single-phase shaded pole motors, use a system in which a bobbin is wound with the conductor and is then placed on part of a core for a laminated yoke to be pressed into a laminated stack. Such an arrangement is shown in U.S. Pat. No. 5,729,071, incorporated herein by reference. However, these methods are not applicable to the single-piece, salient pole laminations used in switched reluctance machines, since the bobbin(s) could not be inserted into the stator.
According to one aspect of the present invention there is provided a winding for a reluctance machine comprising a single coil having opposed side portions, each having first and second ends, a first end loop connecting the first ends of the side portions, and a second end loop connecting the second ends, at least one of the first and second end loops defining an arch between the side portions such that an enclosed aperture is defined by the winding when viewed generally along the direction of the side portions. According to one embodiment, the winding of the present invention is of a shape that can be preformed and fitted into a stator.
Preferably, both the first and second end loops define arches between the side portions which project in opposite directions and, together, define the aperture. However, only one of the end loops needs to project in an arch to define the aperture, the other end loop remaining substantially within the plane of the side portions.
According to one form of the invention the coil is shaped to enable it to be connected to an electronic controller and tested before it is inserted into the stator of a switched reluctance machine.
Preferably the switched reluctance machine is a single-phase machine with a single coil which embraces all the poles.
Embodiments of the invention provide a winding shape which can be completely formed outside the stator and then inserted into the stator without further deformation being required to allow the rotor to be assembled. Only a single coil is required to magnetize the magnetic circuit of the machine. If the wire from which the coil is made is sufficiently rigid, the coil will be able to retain its own shape without requiring further support. In many cases, however, it will be found preferable to wind the coil onto a specially shaped bobbin and insert that bobbin into the stator. The bobbin may also usefully have mounted to it a circuit board, for example an electronic controller for controlling excitation of the winding, or the components of a rotor position transducer (RPT), providing feedback for control of the machine. The RPT may provide signals indicative of rotor position or rotor movement.
When the winding has bobbin, it is possible to shape the bobbin so that it fills the gaps between stator poles. Exposed poles create air turbulence as the rotor rotates relative to the stator. By smoothing the surface between the stator pole faces by means of the shape of the bobbin, windage losses due to the turbulence are reduced.
Preferably, the winding includes means for mounting at least one magnet on one or both of the end loops.
In one particular embodiment the aperture provides clearance, when assembled within a switched reluctance machine stator, for a rotor to be inserted at least from one end.
The invention also extends to a method of assembling a stator for a reluctance machine, the stator defining stator poles and including a winding as defined above, the method including introducing the second end loop of the winding into a space in the stator between the stator poles, and tilting the winding such that the side portions are arranged substantially in parallel with the axis of the stator for excitation of at least some of the stator poles.
The coil may be arranged in the stator so that the coil sides partially or completely fill a space between adjacent stator poles.
Also according to an embodiment of the invention there is provided a switched reluctance machine including a winding as defined above. In this regard, the invention is particularly applicable to the machine of U.S. Pat. No. 5,043,618 (mentioned above) in which two poles of like polarity are grouped together.
The invention can be put into practice in various ways, some of which will now be described by way of example with reference to the accompanying drawings, in which: